The “Denture Sensor”

Extract from Chapter Two – Sense and Sense Ability:

The Denture Sensor

I have revealed some of the intrinsic strengths and weaknesses of our senses, and next I will explore the part played by our imagination. Before moving on, I want to share a bit of fun with you. To be creative, it helps to be able to connect with the child part of oneself, the part that knows how to play, indeed companies like Google now recognise this, and provide their staff with suitable play areas. I was fortunate to work in a research environment that tolerated a few wacky diversions during breaks in the work.

Although optical fibre was pioneered for use as a communications medium, we were always looking for other novel applications of the technology. We experimented with a variety of incredibly sensitive sensors for sound, vibration, temperature, rotation and other physical phenomena, all based on fibre optics, and colleagues and researchers from other departments soon came to expect to be shown all kinds of sensing devices with very high sensitivity.

In most laboratories you would find oscilloscopes, though this is less true today as computer screens have now largely replaced them. An oscilloscope is a piece of electronic equipment that displays a waveform of some sort, typically a voltage which might be derived from something that is being measured or monitored. Among users it is simply referred to as a “scope”. You may be familiar with the device used in hospitals that displays a patient’s pulse as a waveform on a screen; this is an example of an oscilloscope display. In our laboratory almost everyone used a scope; people were very familiar with seeing a little bright spot scanning rapidly across a screen, tracing out some important waveform or other.

One day while looking at an oscilloscope screen in the distance, I noticed that when I clicked my teeth, I simultaneously saw a brief oscillatory waveform on the oscilloscope. How could my jaw actions cause a waveform on the oscilloscope? This fascinated me and after a bit of experimentation I worked out that the effect was caused by my eyeballs bouncing! To be more precise, as I clicked my teeth, the vibration was transmitted through my skull to my eyeballs and caused them to rotate slightly, oscillating about the vertical axis. As the spot scanned smoothly across the screen of the oscilloscope (set to scan at 20 milliseconds per division), the point of gaze of my eyes moved up and down slightly. In the absence of any teeth clicking, the image of the horizontally scanning point of light on the oscilloscope screen would normally be imaged onto the retina of my eye as a horizontal line of illumination. But because the direction that my eyes were pointing was bouncing up and down, it drew out an oscillatory waveform on my retina instead. So although there really was no signal going into the oscilloscope and it was in reality “flat-lining”, I personally saw a waveform that appeared to be on the screen, but which in truth was only on my retina.

I asked one of my colleagues to click his teeth while watching the screen, and he was immediately fooled into thinking that I had built a very sensitive microphone. He found it difficult to believe that this was just an illusion, even after I showed him that the oscilloscope was not connected to anything. I immediately began to appreciate the potential for having a bit of fun (and also as a tool for exploring how we respond to new perceptions). I flippantly called it “the denture sensor”, not realizing at the time how appropriate that might be.

Now, having been fooled himself, he was eager to see how someone else would react. So I began a sequence of “behavioural experiments”, in which naive colleagues drifting into our department would be shown our “denture sensor”. Previously gullible subjects eagerly gathered round and pretended that they too could see the waveform on the ‘scope when the new victim clicked his teeth, whereas in reality only the “clicker”, (i.e. the person clicking their teeth together), would see a waveform. A plausible story was concocted to explain why the subject did not see any waveform when others in the room clicked their teeth. We told them that we used a microprocessor to recognise and filter out the signals from all those people who had previously been tested (At that time, there was so much publicity for the recently invented microprocessor, that people were prepared to believe that it was capable of almost anything). I set up a dummy “sensor” next to the ‘scope, using a coil of optical fibre wound onto a cardboard tube and this all added credibility to the idea.

Those who had previously been taken in by the illusion were absolutely delighted to see each new victim fooled (I think it made them feel so much better about their own earlier gullibility). New subjects reacted in different ways: Almost everyone was fooled initially, and almost no-one worked it out without being given several major clues. Once people had it fixed in their mind that they were seeing a signal on the scope – rather than in their eyes and mind – it was surprisingly hard to convince them otherwise.

Because the effect is the result of an angular change in the pointing direction of the eye, the waveform appeared to get bigger, as the viewing distance from the screen increased. In reality the observed eye bounce waveform stayed constant size as it had nothing to do with the ‘scope, but the size of the scope screen diminished with distance, so in comparison the signal seemed to increase in size with distance. Now almost everything else in Physics decreases the further away we are from it, (typically inversely to the square of distance), so this was a very disturbing observation for our subjects, yet they still could not recognise that the waveform was only in their heads.

Only one single person got it first time, and he had previously observed something similar. The rest needed to have it explained to them. One, seeing it as a challenge to his intelligence insisted: “Don’t tell me, I will work this out if it kills me”, and of course we had to intervene for health and safety reasons after he had failed to work it out for a worryingly long time! Another stormed out and avoided visiting us for some weeks. You may think we were cruel, but play is a vital tool in creating an inventive environment (at least that is what I had planned to argue in my defence if the management discovered what we were up to). Those who didn’t bolt learned something important too. They learned just how easy it is to misinterpret an observation.

One day I demonstrated it to the labs Chief Scientist and he too was initially fooled and then very intrigued. A few weeks later, to my horror he swept in through the doors accompanied by a large group of dignified and respectful Japanese visitors, saying: “Richard, I have brought the head of Sony Electronics for you to show him your clever sensor”. Now the British sense of humour has always been difficult to translate into Japanese, especially using words like “denture”. I felt pretty sure that someone’s blood might be have to be ritually shed if I were to humiliate the big boss in front of his entire entourage of more junior staff, (who incidentally seemed unable to stop repeatedly bowing respectfully). All I could think to do at that moment was just to quickly explain the whole thing before demonstrating it, which completely eliminated the impact (and the fun!). Frankly, I could tell from the look on the big man’s face that he thought we were a load of idiots, and as he and his disciples swept out through the door, I realized that it was time to cut back on this particular avenue of research and focus on what we were supposed to be doing.

Many years later, on the last working day before the Christmas break, my colleagues persuaded me against my better judgement, to set up the “denture sensor” again so we could entertain ourselves with the reactions of those who had only paid us a visit to eat our mince pies. We had fun catching out quite a few of our visitors, but then who should enter but the managing director himself, who had just dropped by to wish us a Happy Christmas. Sensing the atmosphere of fun, he asked what we were doing, and I felt obligated to give him the full treatment (it was almost a festive holiday after all).

Well, he saw no waveform, it just did not work for him, and he was clearly almost as confused as our previous Japanese visitor. After he left, I speculated that what we had been playing with all this time, really was capable of detecting the presence or absence of dentures. I certainly didn’t have the nerve to ask the director if he wore dentures, or whether they were attached with vibration damping adhesive. That was the last time we played the game. If you want to experiment with the phenomenon yourself, see: How to make your own denture sensor.

You may be wondering why I have related this tale, and how it might be relevant to the topic of this book. Well it taught me two very important things: First; that an incorrect explanation can be very difficult to displace despite new evidence to the contrary. Secondly, it was a clear example of how things that are going on in our head (bouncing eyeballs in this example), are all too easily interpreted as things happening in the outside world, and revealed just how difficult it is to appreciate the difference (in the absence of an additional honest independent observer).